FK506, also known as tacrolimus or fujimycin, is a 23-member macrocyclic lactam and can be isolated from Streptomyces tsukubaensis. FK506 and drugs similar thereto are known to interact with cytoplasmic immunophilin proteins, commonly referred to as FK506-binding proteins (FKBPs), to modify several biochemical reactions (Kang, C. B. et al., Neurosignals 2008).
In particular, FK506 clinically prevents allograft rejection (Kino, H. et al., J. Antibiot. 1987; Kino, H. et al., J. Antibiot. 1987; Fung, J. J. et al., Transplantation 2004) and is used as an immunosuppressant for treating autoimmune diseases such as atopy (Parsons, W. H. et al., Ann. N. Y. Acad. Sci. 1993). FK506 is an important element in signaling of T-cell receptor and suppresses the activity of calcineurin, which suppresses T lymphocyte activity.
In a specific study of the mechanism of immunosuppressive activity of FK506, the chemical structure of FK506 can be divided into two regions: an effector region which binds to calcineurin (Goulet, M. T. et al., Perspect. Drug Discov. 1994; Parsons, W. H. et al., Ann. N. Y. Acad. Sci. 1993; Griffith, J. P. et al., Cell 1995) and a binding site which forms a complex with FKBPs. The FKBP binding site comprises a pipecolate moiety, a tricarbonyl group, and a cyclohexane ring, and plays an important role in complex formation with the FKBP12 protein. This allows the remaining effector region to freely bind to calcineurin so as to form a tripartite complex.
In order to exert the immunosuppressive effect above, it is important that FK506 first binds to the FKBP12 protein. Once the two-part complex is formed, the complex can interact with calcineurin (Goulet, M. T. et al., Perspect. Drug Discov. 1994; Parsons, W. H. et al., Ann. N. Y. Acad. Sci. 1993). The interaction of FK506-FKBPs complex with calcineurin inhibits interleukin-2 mediated by T-cell proliferation, resulting in immunosuppressive action.
FK506 is synthesized by a hybrid PKS/NRPS (polyketide synthase/nonribosomal peptide synthetase) system. The biosynthesis process is carried out using DHCHC (4,5-dihydroxycyclohex-1-ene carboxylic acid), which is derived from chorismate, as a starting material, where DHCHC is extended by 10-step condensation using two molecules of malonyl-CoA, two molecules of methoxymalonyl-acyl carrier proteins (ACP), five molecules of methylmalonyl-CoA, and one molecule of allylmalonyl-CoA (Andexer, J. N. et al., Proc. Natl. Acad. Sci. U.S.A 2011; Mo, S. J. et al., J. Am. Chem. Soc. 2011). Pipecolate derived from lysine by FkbL action is condensed with a linear polyketide chain by NRPS KfbL and is cyclized to produce a macrolide ring. The ring is further modified by a post-PKS modification process such as O-methylation at C-31 caused by FkbM (S-adenosylmethionine (SAM)-dependent methyltransferase) or an oxidation reaction at C-9 by FkbD (P450 hydroxylase) (Motamedi, H. et al., J. Bacteriol. 1996; Shafiee, A. et al., J. Antibiot. 1997).
Recently, the present inventors have established that a post-PKS modification path includes two independent parallel paths via characterization of all FK506 biosynthetic intermediates (Ban, Y. H. et al., J. Nat. Prod. 2013).
In addition to the above immunosuppressive activity, FK506 or derivatives thereof have been reported to have anti-fungal (Nakagawa, H. et al., Clin. Drug Invest. 1996), anti-inflammatory (Migita, K. et al., Curr. Med. Chem. 2003), and neuroprotective and neuroregenerative effects (Gold, B. G. Expert Opin. Invest. Drugs 2000; Gold, B. G. et al., J. Pharmacol. Exp. Ther. 1999). Despite the pharmacological activities above, however, FK506 or derivatives thereof have the immunosuppressive activity, thereby causing a problem such as making it difficult to be used for general patients who require an immune reaction.